Now I found out how to properly get rid of this problem.
Install the AC3 filter v0.66b (can be downloaded from
www.doom9.org)
Go to BSPlayer options - preferences tab:
on General check the "dont load AC3 settings"
on Audio uncheck the "use dedynamic filter"
on Audio set the "for AC3 decoding use" to "--"
Open your movie with AC3 streams in BSPlayer.
Go to Properties - AC3filter tab:
set speakers to 3/2+SW 5.1 channels
Now you should have all your 5+1 speakers working fine. You can play around with equalizer etc. but do not set the "Sample format" to "PCM float" unless you know exactly what u r doing. I did not know and I almost lost my ears :)
Enjoy